A factor which currently limits the contrast resolution of diagnostic ultrasonic imaging systems is the phenomenon of coherent speckle. The formation of speckle is due to interference effects arising from phase variations of the signals accumulated during the imaging process. These phase variations may be caused by diffuse scatterers in the object field, by multiple scattering, by an inhomogeneous propagation medium which distorts the phase of the received wave, or by phase aberation of the imaging system.
Although deterministic, speckle appears as random noise superimposed on the image, and degrades the apparent contrast resolution of the image. It is known that the speckle pattern of an image changes with displacement or rotation of the imaging aperture relative to the subject being imaged. The speckle pattern at a location in a subject also changes as the scattering structure surrounding the location changes. One common approach to reducing speckle involves combining or averaging multiple images having uncorrelated (or partially uncorrelated) speckle patterns, a technique known as "compounding." in general, compounding can be carried out in the spatial, frequency or time domains.
For spatial compounding, the imaging aperture can be divided into a number of subapertures, and signals from the subapertures are averaged. To satisfy the requirement that speckle patterns be uncorrelated, the translation of the subapertures must be more than half size of the subapertures. A disadvantage of spatial compounding is the concomitant reduction of spatial resolution caused by partitioning of the finite imaging aperture. A further disadvantage is that if the loss of temporal resolution cannot be tolerated, then parallel subaperture beam-forming is required, an approach that increases system cost.
Frequency compounding can be accomplished by dividing the bandwidth of the imaging system into multiple nonoverlapped (or partially overlapped) bands. Signals from the different frequency bands are then processed and averaged. A disadvantage of frequency compounding is that the narrow band processing results in loss of both lateral and axial resolution. The well-known frequency dependent attenuation properties of body tissues also limits the effectiveness of frequency compounding at large depths. A further problem is that temporal resolution is degraded, since multiple transmit pulses corresponding to the different frequency bands must be used.
Temporal compounding involves averaging successive frames with one another. Due to the fact that most live tissues move, either actively as part of their function, or passively in response to the respiratory, cardiovascular or gastric movements of nearby organs and blood vessels, the geometrical distribution of scattering structures surrounding an imaging point continually change with time. As a result, speckle patterns in the image also change continually. Thus, compounding images in the temporal domain will reduce the speckle noise. An advantage of temporal compounding over spatial or frequency compounding is that temporal compounding is relatively simple and inexpensive to implement. In addition, it involves little or no loss of spatial resolution. However, temporal compounding generally requires a direct trade-off between improvement in contrast resolution and loss of temporal resolution.